According to the state of the art, strip processing plants, such as those used for metal pickling plants, using liquid baths are designed with each individual bath measuring approximately 100 to 500 mm deep and normally 10 to approximately 40 m long. The reaction speed of the treatment medium on the surface of the strip material being processed is improved by reducing the liquid boundary or barrier layer on the strip surface. To reduce the thickness of the liquid boundary layer on the strip of material, the current trend is now towards the use of shallower liquid baths. The shallow bath depth produces greater bath turbulence due to the higher Reynolds number and a reduction in the thickness of the liquid boundary layer due to the increased shearing effect. The shallow bath depth also results in less sag of the strip material in the bath.
The required average depth of the bath is dependent, in part, on the speed of the strip material passing through the bath. Due to the dragging effect in the liquid produced by the moving strip, the liquid treatment medium is carried along in the treatment tank in the same direction as the advancing direction of the strip. This results in a slant or incline of the liquid surface level with a corresponding increase in the depth of the bath at the strip exit point and the equivalent reduction in the depth of the bath at the strip entry point. At higher speeds (e.g., above a strip speed of approximately 200 m/min with 200 mm mean depth and 20 m length of the bath), the liquid is carried away by suction from the strip entry point into the treatment bath such that the distance over which the strip is submerged in the treatment medium is shortened accordingly. This results in the length of the treatment bath and the tank not being fully utilized. As a result, the mean liquid level of the bath must be raised to avoid the strip entry area being emptied by the suction from the advancing strip. The optimum efficiency of a treatment bath with an open top surface is thus a compromise between the shortened bath length caused by the drag which reduces the pickling effect and the greater mean depth of the treating liquid bath.
Various treatment baths have been designed to eliminate the tensioning devices by providing shallow baths. One example of a bath having a structure to maintain a small mean bath depth at high speeds is by the use of a closed treatment channel. However, the level of the treatment liquid can be higher than the entry and exit points of the strip. The depth of the bath is thus limited by the height of the treatment channel. In order to prevent the liquid from being drawn or sucked out of the treatment channel, the strip exit point from the treatment channel must be sealed hydrodynamically against the back-up pressure caused by the dragging effect. An example of this type of structure and process is disclosed in, for example, EP 0 655 519 A1. The hydrodynamic seal requires a much higher energy input compared to the treatment baths with an open top surface. Furthermore, the high velocity and head pressure caused by sealing the bath at the strip exit point of the treatment channel makes it more difficult to provide the required supply of fresh treatment medium to the strip being treated. The closed channel requires more energy to feed the fresh liquid treatment medium into the channel.
In addition to the optimum contact and surface treatment of the strip of material with treating liquid, it is desirable to minimize the contact of the moving strip with the bottom of the trough. Contact of the strip with the bottom of the trough produces scratches and scrapes and causes smooth or worn areas on the strip, thereby reducing the quality of the finished strip material. Deep treatment baths allow the strip material to be run in catenarian curve without the strip contacting and dragging along the bottom of the trough. However, shallower baths which require at least some sag in the strip forming a catenarian curve result in the strip contacting the bottom of the trough, particularly when the strip is advanced at low speeds. The relation between the amount of the sag in the strip for a given tension and the length between the tension supporting rollers is not a reliable control of the sag to prevent contact with the bottom of the trough. Adjusting the tension of the strip material in the liquid bath can reduce some of the contact of the strip along the bottom, but generally cannot eliminate the contact entirely. Generally, slides, wedges or other support members are placed along the bottom of the trough to minimize contact of the strip with the bottom of the trough.
The feed mechanisms of the pickling treatment baths include various tensioning devices to control the amount of sagging in the strip of material being carried through the treatment baths. Depending on the material being treated and the depth of the bath, the tension of the strip can be adjusted to provide little or no sag.
Contact of the strip material with the bottom of the trough is, in part, dependent on the speed of the strip in the bath. At higher strip speeds, a hydrodynamic sliding film of the treatment liquid forms between the strip and the bottom wall of the trough or the slides and wedges in the bottom of the treatment bath, thereby preventing scrapes or smooth areas on the strip. At low strip speeds, however, the strip scrapes along the bottom wall or over the slides and wedges of the treatment bath such that the surface quality of the strip may be impaired and causing wear of the bottom of the bath or the slides and wedges.
Continuous strip treatment plants include equipment for welding together the individual strips to be treated, and thus, allowing them to be pulled through the treatment baths without a break. In a particular type of strip treatment plant, such as a push-type pickle bath according to EP 0 302 057 B1, suitable equipment pushes the strips through the treatment baths. The strip head is first gripped and then the complete strip pulled through the treatment bath. After the insert procedure, these plants are subject to the same conditions for the treatment bath as described above for continuous strip treatment plants. Unlike the continuous strip plants, however, there is no means of preventing the contact forces between the strip and the bottom wall of the bath. These plants do not provide a way to apply tension to the strip while the strip is being pushed in and pulled out because the strip head and tail are not secured. Thus, sufficient strip tension cannot be applied to minimize contact of the strip with the bottom wall of the trough. In addition, loops or folds can form when the strip is inserted. This looping is caused by friction between the strip and the bottom of the bath or by faulty strip head guidance. The plant operations then have to be halted and the strip reinserted into the treatment bath to resume the treatment of the strip.